1. Field of the Invention
Embodiments of the invention generally relate to methods for the fabrication of photovoltaic, semiconductor, and electronic materials and devices, and more particularly relate to epitaxial lift off (ELO) processes and the thin films and devices formed by these process.
2. Description of the Related Art
Photovoltaic or solar devices, semiconductor devices, or other electronic devices are usually manufactured by utilizing a variety of fabrication processes to manipulate the surface of a substrate. These fabrication processes may include deposition, annealing, etching, doping, oxidation, nitridation, and many other processes. Generally, the manufactured devices generally incorporate a portion or the whole base substrate into the final architecture of the electronic device. For example, a photovoltaic device is often formed on a gallium arsenide wafer which is incorporated as an intimate part of the final photovoltaic device. Epitaxial lift off (ELO) is a less common technique for fabricating thin film devices and materials which does not incorporate the base substrate into the final manufactured devices.
The ELO process provides growing an epitaxial layer, film, or material on a sacrificial layer which is disposed on a growth substrate, such as a gallium arsenide wafer. Subsequently, the sacrificial layer is selectively etched away in a wet acid bath, while the epitaxial material is separated from the growth substrate. The isolated epitaxial material is a thin layer or film and is usually referred to as the ELO film or the epitaxial film. Each ELO film generally contains numerous layers of varying compositions relative to the specific device, such as photovoltaic or solar devices, semiconductor devices, or other electronic devices.
The growth substrates are usually crystalline wafers of gallium arsenide or other Group III/V elements. The growth substrates are very fragile and expensive. The growth substrates are so expensive as to be commercially cost prohibiting if incorporated into the finished ELO film or device. Therefore, once the ELO film has been removed, the growth substrates are cleaned, treated, and reused to manufacture additional ELO films. While reusing the growth substrates reduces some cost, the process of refurbishing a growth substrate for each fabricated ELO film is still quite expensive. The growth substrates must be refurbished even if the ELO process does not yield a commercially viable ELO film. Also, since the growth substrates are quite fragile, the likelihood of chipping, cracking, or breaking a substrate increases with each additional step exposed to the growth substrate during the ELO or refurbishing processes. Furthermore, each of the growth substrates has a finite life expectancy even if the substrate is not damaged during the fabrication processes.
While the expense of growth substrates may be one factor which has contributed to the lack of commercial utilization of the ELO process, other factors have also plagued the use of this technique. The overall ELO process has always been a cost prohibiting technique for commercially producing the thin ELO film devices. The throughput is quite low since current ELO processes provide transferring a single growth substrate through many fabrication steps while producing a single ELO film. The current ELO processes are time consuming, costly, and rarely produce commercial quality ELO films.
Therefore, there is a need for a method for growing epitaxial film stacks by ELO processes, and a need for the method to have a high throughput and to be more effective, less time consuming, and less expensive than currently known ELO processes.